1. Field of the Invention
The invention is generally related to monitoring the condition of a polymeric material or a chemical fluid using a frequency dependent electromagnetic sensor. More particularly, the invention is directed to a dosimeter device which has reproducible electrical permittivity characteristics for a given treatment history that are matched to physical attribute characteristics of a polymeric material or chemical fluid being monitored, whereby the deterioration of the physical attributes of the polymeric material or chemical fluid can be discerned from a permittivity measurement, real and/or imaginary component, of the dosimeter.
2. Description of the Prior Art
U.S. Pat. Nos. 4,710,550 and 4,723,908 to Kranbuehl, both of which are herein incorporated by reference, describe a probe and its method of use in monitoring the changing processing properties of a polymeric resin as it is fabricated into a part in an oven, press, or autoclave. The probe is preferably thin and flat, and includes an array of electrode lines in an interdigitated pectinate configuration on a substrate. The lines are made of a conductive material, such as tungsten, gold, copper, platinum, palladium, chromium, or alloys of the same. The lines on the Kranbuehl probe are less than 10 mils (one thousandth of an inch) apart, less than 20 mils wide, and form the two terminals of a capacitor. The substrate in the Kranbuehl probe has a low loss tangent over a frequency range of one Hertz (Hz) to approximately 10 megaHz. In particular, the substrate has a conductivity which remains below approximately 10.sup.- .sup.7 ohm.sup.-1 cm.sup.-1 over its range of use, which in thermal processing of polymers can range between 0.degree. C. to 500.degree. C. Exemplary substrates include A.sub.2 O.sub.3, glass, ceramic, and low loss polymer film (e.g., Kapton.RTM.).
In operation, a material is placed on the substrate in contact with the array of lines. A voltage is placed across the two electrically isolated arrays. An electric field between the lines passes up and through the material which is in contact with the probe. Hence, the probe utilizes the fringing effects of the electric field to measure the dielectric properties of the material, as well as the electric field which passes through the small amount of material which is directly between the lines. Measurements are preferably made with an impedance analyzer which includes low noise, automatic bridges that can span up to six decades or more in frequency. The impedance analyzer measures the opposition that a material presents to an alternating current in terms of the complex ratio of the voltage to the current. This relationship is set forth in Equation 1 EQU Z*=V(.omega.)/I(.omega.) Eq. 1
where Z* is the complex impedance. The output of the impedance analyzer is representative of the magnitude and time shift of the voltage relative to the current.
The Kranbuehl patents discuss in detail how the properties of the material can be represented as an equivalent circuit of a resistor and a capacitor in parallel and how the material's electrical properties acquired using the impedance analyzer are best understood in terms of its complex permittivity (.epsilon.*), an intensive property of the material which has both real and imaginary components. Equation 2 presents the complex permittivity calculation. EQU .epsilon.*=.epsilon.'-i.epsilon.'' Eq. 2
The complex impedance sensed by the impedance analyzer can be modeled as a parallel circuit which includes both a resistor and a capacitor. As explained in the Kranbuehl patents, measurements of the equivalent parallel circuit components of the complex impedance, e.g., the capacitance C and the conductance G, are used to calculate .epsilon.*. Using either bridge or time-domain techniques, the real and imaginary components of the material's macroscopic impedance Z* is determined as a function of frequency. The complex permittivity .epsilon.* can be calculated knowing the capacitance of the material (C.sub.p), the capacitance of the probe without the material (C.sub.o), and the conductance of the material (G.sub.p) as set forth below in Equations 3 and 4. EQU .epsilon.'=C.sub.p /C.sub.o Eq. 3 EQU .epsilon.''=G.sub.p /.omega.C.sub.o Eq. 4
Both the real and imaginary components of .epsilon.* have dipolar and ionic components as indicated by Equations 5 and 6. EQU .epsilon.'=.epsilon.'d+.epsilon.'i Eq. 5 EQU .epsilon.''=.epsilon.''d+.epsilon.''i Eq. 6
Understanding the contribution of the dipolar mobility and ionic mobility components can provide an understanding of the physical nature of the material being analyzed.
Kranbuehl et al., Am. Chem. Soc., Los Angeles Meeting, Polymeric Materials Science and Engineering Division, Sept. 1988, pp. 839-843, reported that the frequency dependent output of electromagnetic sensors (FDEMS), like those discussed in the Kranbuehl patents, can be used in life monitoring, whereby output could be used to discern the molecular state of composite and polymeric structures as they physically change during use due to extended exposure to chemicals, stress-strain extremes, temperature extremes, high energy radiation, and atomic oxygen. On a molecular level these environmental effects change the chemical structure, cross-link network and morphology of the polymer, which, in turn, changes the structure's toughness, strength and point of failure. It was particularly observed that the real component of permittivity, .epsilon.', remained stable for a short period of time at an elevated temperature and then declined rapidly. This indicates that the complex permittivity measurements made with the Kranbuehl probes could be used as a qualitative indicator of thermal degradation of a material.